Many analytical instruments require a powder sample: to control the shape and/or volume of the specimen; to increase the surface area of the specimen; to increase the statistical representation of a specimen when samples are not homogeneous with regard to the characterized property; and/or to increase the statistical representation of the specimen spatial orientation when the properties being characterised are not equivalent in different viewing directions.
In one embodiment of the invention, the instrument is an X-ray diffraction apparatus, as the invention is particularly suited to X-ray instruments that utilize small X-ray beams, such as non-focused or miniature X-ray diffraction instruments.
The powder sample handling technique would also find application with instruments using techniques other than X-ray diffraction. Non restrictive examples of such application are listed herein:
X-ray fluorescence: the analysis consists in measuring the X-ray emission spectrum of the sample when the sample is irradiated by X-rays. If selected energies or white (continuous) X-rays are used to illuminate the sample, inner electrons shells of the constitutive atoms can be excited and emit X-ray radiation at characteristic wavelengths. Measuring the emission spectrum allows identifying the nature and abundance of the constitutive atoms. The powder handling system would be used to insert the sample in the analysis region and randomize the sample during analysis. This would enable complete automation of the analysis with no need to prepare pellets of the powder sample usually done.
Infrared absorption spectroscopy: the analysis consists in measuring the absorption (attenuation) of infrared light passing through a sample depending on the wavelength of the radiation. Chemical bonds in the sample can vibrate at particular frequencies (i.e. with particular energies) and consequently can absorb particular electromagnetic energies. As a consequence, measuring the energies (or wavelength) that are absorbed by a compound allows identification of chemical bounds and in turn identification of the compound. Powder samples are typically used and are ground to fine grain (<2 μm) to limit scattering of incident light. The sample is analyzed in a solid or liquid matrix for index matching. The powder handling technique could be applied to either dry sample, or preferably powders sample in suspension in an appropriate liquid. The sample handling technique would ease manual loading of the sample and enable automatic/robotic operation of the infrared spectrometer. It would also allow randomization of samples and analysis of large quantities of material, or analysis of a stream of material.
Raman spectroscopy of powder sample: Raman spectroscopy consists in the measurement of the wavelength of backscattered radiation of a sample illuminated by a monochromatic radiation usually produced by a laser. Identification of particular wavelength shifts (Raman shift) allows identification of the compound, based on its particular molecular vibrations and/or its crystalline vibrations. The powder handling system can be used in conjunction with a Raman spectrometer to allow automatic loading of powder in the spectrometer, randomize the sample during analysis, and analyze a larger quantity of powder that would be analyzed by conventional techniques.
Microscopic imaging of powder sample: The powder handling system would be used to load the sample in the imaging region of a microscope. Imaging can be done in reflection or transmission mode, on dry powders or with the sample placed in suspension in a liquid.
Imaging particle size analyzer: The powder sample is vibrated for insertion in the imaging region consisting of two windows separated with a distance of the order of the diameter of the largest grains of powder. The vibration amplitude is adjusted so that the grains of powder have a level of excitation spreading them apart. When the vibration is stopped, the grain motion stops. An image of the sample is then collected, in reflection or, preferably, in transmission mode, using a camera equipped with high magnification optics or mounted on a microscope. The image collected is transferred to a computer equipped with an image analysis software that characterizes the size and shape of the grains observed on the image. The sample is then vibrated to randomize the sample. Vibration is stopped and a new image is acquired. After analysis of a sufficient number of images, size and/or shape distributions of the sample are obtained by summing the data of individual image analysis.
However, one of the most demanding analytical techniques, with regard to preparation of powder sample for analysis, is XRD.
XRD, the most common technique for studying crystal structures, is applied in many fields such as geology for identifying minerals, material sciences for studying materials structures and quantifying structural strain, biochemistry for studying macromolecular structures and identifying pharmaceutical compounds, archaeology for investigating localities and processes of fabrication of artifacts, as well as many other applications.
XRD relies on the measurement of the angles at which crystalline matter constructively reflects X-rays from a set of atomic layers defined by the crystal structure. Each crystal structure has a set of possible reflections that occur when the crystal is appropriately oriented in an X-ray beam. Measuring all the possible reflections of a sample in an angular range allows determination of the sample crystalline structure, or the actual sample identification based on its crystalline structure.
Performing an XRD analysis requires exploring all the possible orientations of a crystal and measuring the conditions at which reflections occur within a practical angular range. One approach is to use a single crystal that is rotated in an X-ray beam to expose all possible orientations, but this is mechanically complex, and in terms of size for remote locations, is very impractical.
The most common approach of XRD, called powder diffraction, uses powder material, or solid polycrystalline material, to create a specimen that offers all possible crystalline orientations without requiring complex sample movement. It is assumed that the specimen is composed of a large number of crystals, identical in structure and oriented in all directions so that all of the possible orientations leading to the reflection of X-rays are statistically well represented. Quality powder diffraction data can only be obtained with fine grained powders (less than about 10 μm in diameter), because relatively fine grains such as these lead to a better statistical representation of the crystal orientation within the finite volume exposed to the incident radiation. This is the case for all powder diffraction instruments, but is particularly important for instruments for which the volume of material under X-ray illumination is very small, such as miniature instruments and instruments that are not based on focused/parafocused geometries. These instruments are very sensitive to the grain size of the powder, with the quality of the data being dramatically altered as the grain size increases.
Grinding the material down to an ideal grain size is sometimes impossible, and conditioning the sample for analysis is often time consuming and labor intensive. Further, for operation in remote or extreme environments, neither of these approaches are acceptable.